A DEFENCE barrister continues to hammer the prosecution's key witness in the McCulkin murder trial.

Barrister Dennis Lynch started questioning Peter Hall late on Thursday morning regarding his knowledge of the alleged killings of Barbara McCulkin, 34, and her 13 and 11-year-old daughters Vicki and Leanne.

The trio disappeared from their Highgate Hill home on January 16, 1974.

Torbanlea man Garry Reginald "Shorty" Dubois, 69, has pleaded not guilty to one charge of deprivation of liberty, two of rape and three charges of murder.

Mr Hall on Thursday told a Brisbane Supreme Court jury that Mr Dubois confessed he raped one sister and that he was present when Mr O'Dempsey raped the other girl and murdered the family.

However, Mr Hall was forced to concede that he lied about this conversation and his involvement in a nightclub fire in 1973 when he appeared before the Crime and Misconduct Commission in 2014.

"I admitted lying the first time I came up here," Mr Hall said this morning under questioning.

"I told the truth (at the committal hearing), but I didn't' tell the truth the first time (at the Crime and Misconduct Commission in 2014)."

ARM Newsdesk reported yesterday that Attorney-General Yvette D'Ath signed indemnity documents ensuring Mr Hall cannot be prosecuted for perjury or for crimes he admits to while giving testimony in this case.